1. Field of the Invention
This invention relates generally to medical apparatus and, more specifically but without limitation, to an electrode apparatus for interacting with the brain of a subject.
2. Description of Related Art
Brain electrical activity (BEA) is a readily accessible and reliable index of brain state and function. It allows distinction between both normal states, such as wakefulness or sleep and its different substates, such as NREM and REM, and abnormal states, such as the ictal and inter-ictal substates of an epileptic brain. BEA is also an important tool for localizing an anatomical substrate of a particular physiological function and for understanding how the function correlates with, or is integrated into, behavior. Brain electrical activity plays a critical role in the evaluation and treatment of pharmaco-resistant, or intractable, epilepsy, movement disorders, and other neurological diseases. Furthermore, BEA is the basis for real-time automated detection and prediction of the clinical onset of seizures as disclosed in U.S. Pat. No. 5,995,868 issued Nov. 30, 1999 to Ivan Osorio et al, which is incorporated herein by reference. Output from such real-time automated detection and prediction of the clinical onset of seizures provides the cue for delivery of suitable therapeutic means for automated blockage of seizures by either contingent or closed-loop therapy. Closed loop therapy would benefit a significant number of persons with pharmaco-resistant epilepsy, thereby improving quality of life and decreasing morbidity, mortality, and cost of care associated with epilepsy. In addition to epilepsy, recordings and analysis of BEA also plays an increasingly important role in the diagnosis and control of a wide variety of other brain disorders.
While recording of electrical activity directly from the cortical surface provides more information than scalp recordings, the current methodology, techniques and devices/probes have important limitations that must be overcome if further advances are to be made in the field of neurosciences in general and of epilepsy in particular. The following brief technical overview provides the anatomical and physiological basis for the understanding of such limitations and for the rationale and justification behind the invention described herein, whose objectives are to enhance signal (BEA) quality and information content, and to control/prevent abnormal or undesired state changes.
The following is intended to provide an understanding of the environment on the present invention:
a) The folding of the cortex of the human brain causes the cortex to be a very complex generator;
b) volume conduction and signal dispersion, which impair the ability to precisely localize the site of origin of electrical activity in the brain, are of high degree at the superficial layers of the cortex (which correspond at most to approximately 0.5 mm of the entire cortical thickness) unlike in deeper parts of the cortex where voltage gradients of up to 10 mV/mm are found in the vertical direction; more specifically, the weight of synaptic relations in the cerebral cortex is in a vertical direction, normal to the cortical surface;
c) deeper cortical layers, especially layer V, are intimately connected with seizure generation and propagation;
d) seizure patterns may be restricted to certain cortical layers and multiple seizure patterns may occur superposed, without influencing one another;
e) signal quality and information content are much lower at the surface than at deeper layers of the cortex. Spikes, the single most important index of potentially epileptogenic tissue, reach maximum voltage in the depths of the cortex where they originate and from where they propagate towards the surface. Similarly, spikes have higher frequencies and richer morphology (higher information content) at the depths of the cortex than at the surface of the cortex. Moreover, information about the time relationship between spikes, which is critically important for detection and control of state change of the brain, is highly accurate at the depths of the cortex and very limited at the surface of the cortex; and
f) cortical columns have a diameter of 200-500 nm. Cortical macrocolumns have a diameter ranging between approximately 0.5-3 mm and a height of approximately 2.5 mm. A cortical macrocolumn typically contains approximately 105-106 neurons. The distance to which cells in the macrocolumn send collaterals is approximately 3 mm, which provides an indication definition of their spatial scale. Minicolumns having a diameter of approximately 20-50 μm has been proposed as a basic functional unit of neocortex; these dimensions can be defined by the characteristic lateral spread of axons of inhibitory neurons. A minicolumn spanning the entire cortical thickness contains approximately one hundred ten neurons lined up along its axis, whereas a striate cortex contains approximately two hundred sixty neurons. Simultaneous interactions can be expected to take place at other spatial scales (for example between neurons, minicolumns, corticocortical columns, cytoarchitectonic regions, etc. An electrode having a tip diameter of 10−3 cm and placed closed to the body of a neuron records activity contributed by approximately one hundred neurons
These observations lead to the conclusions that the superficial cortex acts as a filter for the activity of the deeper layers of the cortex and that no prediction as to the site of origin, direction, speed and range of propagation of electrical activity can be made from the cortical surface with a useful degree of accuracy. The intricate cytoarchitecture and gross morphology of the cortex (with folding as its most striking feature) underscore the need and importance to simultaneously record/sense and control activity intracortically and from its surface (epicortex) to improve signal quality, information content, and temporo-spatial resolution. Accomplishing this important objective, requires electrodes/devices not currently available since existing intracranial electrodes, either subdural or depth, have several significant limitations, which limitations as hereinafter described also apply to electrical stimulation of the cortex. Such limitations can be briefly described as follows:
Poor Spatial Resolution
The metal contacts of prior art strip-type electrodes 2, see FIG. 2A, and prior art grid-type electrodes 3, see FIG. 2B, record activity only from areas of the most superficial cortex and which are naturally exposed within the cranial cavity, such as the top or crown of gyri, while approximately two-thirds of the cortex is not accessible to direct surveillance using existing electrodes. Naturally exposed cortices 4 of the gyrus and naturally unexposed cortices 5 of the gyrus are illustrated in FIG. 1. In fact, only about one-third of the cortex and its associated electrical activity are accessible to grid-type or strip-type electrodes. Strip-type and grid-type electrodes that are typically placed on the surface of the cortex. A strip-type electrode 2 can only be used on the exposed cortices 4 of the gyrus.
Depth electrodes 6, see FIG. 2C, another type of sensing devices, are too large for use in the cortex, being better suited for recording from, and stimulation of, subcortical structures such as the amygdala and hippocampus located medially (internally) in the temporal lobe and in other subcortical nuclei such as the thalamus, which differ in several important aspects from the neocortex. Furthermore, even if the diameter of depth electrodes were to be reduced, their ability to localize and track spatio-temporal spread (due to the fact that they would provide only one-dimensional information) would be impaired. Also the possibility of radial migration of these type of electrodes (with consequent loss of the desired target) would remain unaddressed. As a result, prior art recording devices and methods are unable to:                (i) detect, from the surface without contamination or distortion, electrical fields generated below the cortical surface (or intracortically) especially from the deeper layers where generation of electrical signals is most active. Moreover, signals recorded from the surface are highly filtered, which results in shape- and phase-distortions. As a result, signals acquired with prior art devices provide limited and poor quality information about cortical dynamics;        (ii) detect changes which may be close to the recording device but which project only within a very small solid angle;        (iii) precisely localize the critical or primary region from where physiological or pathological activity of interest is generated because it is transmitted with delay, phase distortion, and loss of amplitude and information content to the top of a gyrus. These limitations are due, in part, to the fact that the amplitude of an electrical signal decays as a function of the distance from where it is generated; in other words, the electrical potential at a point behaves according to V˜q/r where voltage, V, is proportional to charge, q, and inversely proportional ro r, which is the distance between the generator and the recording electrode. Furthermore, a solid angle, whose magnitude determines the magnitude of an electrical field in a volume conductor as viewed from the surface along the inner walls of the gyri, is virtually zero. More specifically, signals from exposed cortices 4 can be recorded because electrical activity from these surfaces 4 form a measurable solid angle at the recording electrodes as indicated by the angle θ designated by the numeral 7 in FIG. 3, while unexposed cortices 5 do not form a measurable solid angle as indicated by attempting to measure brain electrical activity with a depth electrode 8 from the shaded region designated by the numeral 9 in FIG. 3. Any electrode can record activity only from the surfaces whose solid angle is large enough to be measured by an electrode. The size of the solid angle is a function of the position/orientation and size of the neuronal pool in relation to the electrode used to record its electrical activity. In other words, electrical activity along those walls, which constitute a large portion of the gyrus, is either undetected or inadequately recorded at the surface. The inability to resolve potentially discrete but relevant changes in BEA with high spatial accuracy negatively impacts scientific progress and compromises clinical care in the important areas of epilepsy, neurology, psychiatry, and mapping of cognitive or other functions of the brain; and        (iv) extract three-dimensional information since electrical activity is generated and also integrated in three dimensions. Existing devices provide only one-dimensional or two-dimensional information;        
Poor Temporal Resolution
Due to the inability of existing devices and methods to record from the depths, walls, and other unexposed surfaces of cortical gyri, there is a variable delay (often reaching infinity) until the electrical activity generated from the unexposed surfaces reaches the crown of the gyri where the recording device is traditionally placed. This translates into the inability to detect, in a timely manner, brain state changes, whether normal or abnormal, unless the changes occur in the immediate vicinity of a contact and the solid angle is large;
Inability to Track Origin and Spatio-Temporal Evolution or Spread of the Signal
Existing strip/grid or depth electrode designs do not allow the simultaneous recording of signals from exposed epicortical (surface) and intracortical regions or from the depths, walls, and other unexposed surfaces of cortical gyri, providing either only one-dimensional or two-dimensional information, thus hampering the ability to localize the origin and track the spatio-temporal evolution of brain state changes, either normal or abnormal, with a useful degree of precision. As opposed thereto, the present invention disclosed herein provides the capability of obtaining reliable three-dimensional information; and
Low Signal Amplitude, Spatial Instability, Poor Contact of Strip/Grid Electrodes, and Difficulties with Target Acquisition
The stability and degree of contact between the cortex and the recording surfaces of existing recording strip- and grid-electrodes, is limited and inadequate, and usually further decreases as a function of movement and positioning of a subject's head. As a result, the amplitude of the signal is low and of poor quality because when recorded from the cortical surface it depends to some extent on the amount of cerebro-spinal fluid, which acts as a shunt upon the surface of the brain, and the firmness with which the electrode rests upon the cortical surface. Signal degradation occurs because the recording surfaces are not anchored in close contact and in a fixed position relative to the underlying cortex but, instead, “float” over the cerebro-spinal fluid. Also, since the strip- or grid-electrodes in which the recording contacts are embedded enter the cranium at an angle due to the manner in which they are tethered, the contacts closest to the point of entrance are often not in contact with the cortex, while those farthest away from the point of entrance tend to move vertically and laterally, either flapping or fluttering, thereby compromising the quality of both the cortical signal and control capabilities. Furthermore, a strip-electrode inserted through a burr hole often kinks, bends or twists, preventing it from recording from desired regions while potentially increasing the trauma to the cortex, since electrode re-insertions may be required. More specifically, prior art macro-electrodes, such as strip electrode 2, grid-electrode 3 and depth electrode 6 include electrode contacts 10 for recording electrical potentials. However, such prior art electrodes leave much to be desired. For example, a prior art strip electrode is typically inserted into the skull through a burr hole 11, see FIG. 5. The presence of kinks 12 or electrode hanging contacts 13 contribute to contaminated signals, such as that illustrated in FIG. 4B as compared to a good signal as illustrated in FIG. 4A.
Another factor which degrades cortical signals and may considerably limit the ability to control state changes using physical, chemical or other means is the layer of cerebro-spinal fluid (CSF) interposed between the cortex and the device or contact; with prior art electrode design, the amount of CSF under the device may be considerable, with certain positions of the head. This layer of CSF shunts electrical currents (in the case of electrical stimulation), or dilutes/washes out chemicals or medicaments delivered to the underlying cortex to control/prevent undesirable or unsafe state changes. Those skilled in the art appreciate that the invention disclosed herein can be used not only for the detection but also for the control/prevention of undesirable state changes since electrical stimulation and induction of temperature changes, among others, can be effected through some of these devices.
What is needed is a mechanism that provides proper continuous close contact between the cortex, both exposed and unexposed, and recording devices thereby considerably increasing the information content of the signal and decreasing noise; that minimizes displacement and flutter or flap of the recording contacts or surfaces thereby limiting recording and/or control signal degradation and loss of target acquisition; that increases the spatial and temporal resolution by providing three-dimensional information to thereby allow for more precise and timely localization of the site of origin and time of onset of brain state changes; that allows more precise, timely and efficacious therapeutic intervention control of brain state changes; that obviates, in certain cases, the need to expose the brain through burr holes or craniotomy thereby minimizing the risk of damage to the cortex, hemorrhage, infection and reducing surgical and anesthesia time; and that improves efficacy by increasing spatial selectivity of stimulation and number of targets resulting in less trauma to brain tissue; that performs functions with a single device that presently requires two or more prior art devices.